The separation of 1-butene from low molecular weight hydrocarbon mixtures is an important operation in the chemical and petrochemical industries. Catalytic cracking and steam cracking are among the most common and large scale processes leading to these mixed hydrocarbon streams. In the production of methanol to olefins, mixed butene streams are also produced in significant amounts as by-products. These butene streams are typically comprised of both structural and olefin isomers. The need to recover 1-butene from these streams, in particular, is one of high economic significance in providing clean feeds for subsequent processes, such as polymerizations where 1-butene is an important co-monomer in the reactions. However, despite the close proximity in boiling points between 1-butene, trans-2-butene and cis-2-butene, these components are presently separated through a combination of catalytic and super fractionation distillation. The large size of the columns and the energy intensity of such distillation processes have, however, created large incentives for alternative means of effecting these separations in a more energy-efficient and cost-effective manner.
Some of the leading alternatives to distillation involve the use of adsorbents that exploit their ability to selectively adsorb some of the components from the mixture. This has given rise to various forms of pressure and temperature swing adsorption (PSA/TSA) processes in which the mixture is first contacted with an adsorbent material under conditions where one or more of the components are selectively removed. The loaded material is then typically exposed to a lower pressure and/or higher temperature environment where the adsorbed components are released and recovered at a higher purity level. Economic viability requires adsorbent materials that can deliver high separation selectivity, high adsorption capacity, and short duration cycles. An additional and critically important requirement is that the material should not catalyze chemical reactions that might lower the recovery of the desired components and/or render the adsorbent inactive.
Among the adsorbents which have been proposed for the recovery of olefins from hydrocarbon mixtures are ion exchange resins, mesoporous solids, activated carbons, and zeolites. Ion exchange resins and mesoporous solids usually exploit equilibrium adsorption properties in which some of the components are preferentially adsorbed over suitably dispersed chemical agents. They principally rely on the adsorption affinity of cationic active centers such as Ag and Cu ions for the double bond in the olefins (e.g., propylene). The characteristic time associated with the adsorption cycle is that required to bring the mixture close to thermodynamic equilibrium with the adsorbent. Since these materials rely on adsorption equilibrium properties, the diffusion rates of the various components within the adsorbent do not influence the selectivity of the separation process. Rapid diffusion of the species into the adsorbent material is, however, highly desirable in order to speed up the contacting of the species with the adsorption sites and thus lead to adsorption/desorption cycles that have a short duration. Activated carbons and zeolites, on the other hand, typically resort to a combination of adsorption affinity and diffusion control. The diffusional effects in these cases, which are exploited advantageously, are usually a consequence of the small pores that make up these high surface area carbons and zeolites. Two related cases of diffusion control are of interest here. In one extreme case, the separation is achieved by totally excluding the diffusion of some of the components into the adsorbent. The second case exploits a sufficiently large difference in diffusion rates to allow the preferential uptake of some of the components within a predetermined adsorption time. This is typically referred to as a kinetic-based separation scheme. Thus, carbons are usually activated to very high surface area forms in order to provide textural properties and pore sizes that maximize the number of adsorption sites per unit mass of the material while selectively controlling diffusional transport into the structure. In many applications, aluminosilicate and silicate zeolites have become even more attractive than activated carbons because of the ever increasing possibilities afforded by new synthetic routes, which allow for a more flexible and precise control of chemical composition, pore size, and pore volume. The tetrahedrally coordinated atoms in these microporous materials form ring structures of precise dimensions that selectively control the diffusional access to the internal pore volume.
Eight-membered ring zeolites, in particular, have been actively investigated for the separation of small molecular weight hydrocarbons because they possess window sizes that are comparable to molecular dimensions and because they can provide high adsorption capacities. A typical example is the Linde type A zeolite which is characterized by a set of three-dimensional interconnected channels having 8-membered ring window apertures. The effective size of the windows can be controlled by appropriately selecting the type of charge-balancing cations. This has given rise to the potassium (3A), sodium (4A), and calcium (5A) forms, which have nominal window sizes of about 3 Å, 3.8 Å, and 4.3 Å, respectively. Thus, for example, EP-B-572239 discloses a PSA process for separating an alkene, such as propylene, from a mixture comprising said alkene and one or more alkanes by passing the mixture through at least one bed of zeolite 4A at a temperature above 323° K to preferentially adsorb said alkene and then desorbing the alkene from the bed. EP-A-943595 describes a similar process in which the zeolite adsorbent is zeolite A having, as its exchangeable cations, about 50% to about 85% of sodium ions, about 15% to about 40% of potassium ions and 0% to about 10% of other ions selected from Group IA ions (other than sodium and potassium), Group IB ions, Group IIA ions, Group IIIA ions, Group IIIB ions and lanthanide ions.
In applications involving zeolites, it is well known that the control of window size is critically important for achieving high separation selectivities. For a given zeolite structure type, the effective size of the windows can sometimes be modified by partially blocking or unblocking the windows with pre-selected charge-balancing cations. This provides a reasonable, but not necessarily optimal, control of window size because of the inherent difficulties of precisely placing these cations in a uniform manner throughout the structure. More importantly, the propensity of these cations to promote or participate in undesired reactions can lead to detrimental isomerization, oligomerization, and polymerization reactions of olefins. These reactions not only lower the recovery of the desired components, they are also likely to render the adsorbent inactive. The double bonds in the olefins are particularly prone to attack even by mildly acidic sites and this may severely limit the temperature and partial pressures at which the separation process can be carried out. This issue of chemical reactivity is illustrated, for example, by the work of M. Richter, et al., “Sieving of n-Butenes by Microporous Silicoaluminophosphates,” J. Chem. Soc. Chem. Commun. 21, 1616–17 (1993), where a proposal is made for the use of SAPO-17 (ERI) for separating trans-2-butene from 1-butene and cis-2-butene. They report that SAPO-17 exhibits detrimental catalytic activity even at mild temperatures (395° K). Their work also shows that at 333° K the amount of trans-2-butene adsorbed on SAPO-17 exceeds that of the other isomers by a factor of approximately 7. A separation selectivity factor of 7 does not appear to be sufficient for a selective separation process in which trans-2-butene can be produced in high purity and, more importantly, the key component, 1-butene, is not separated from cis-2-butene.
In an effort to control chemical reactivity more reliably, there is a growing interest in the use of non-acidic, all-silica zeolites. Since these siliceous zeolites require no extra-framework balancing cations, the size of the windows is much more uniform throughout the crystals and largely determined by the crystal structure. Thus, for example, the potential of DDR for separating propane and propylene has been recently reported. See W. Zhu, et al., “Shape Selectivity in the Adsorption of Propane/Propene on the All-Silica DD3R,” Chem. Commun., 2453–54 (1999). This crystalline microporous silicate has a two-dimensional pore system formed by 8-membered rings of tetrahedrally coordinated atoms with a nominal window size of 3.6Å×4.4Å (see Atlas of Zeolites Framework Types, Fifth Revised Edition, pages 108–109, 2001). Diffusion and adsorption measurements on this material indicate that only propylene is able to access the interior of the crystallites. The exclusion of propane from the adsorbent interior was suggested as the basis for a very selective separation scheme. The size of the DDR windows, however, appears to be so close to the effective kinetic diameter of propylene that the diffusion rates are very low and this could lead to undesirably long adsorption and desorption cycles. Similar arguments may limit the use of DDR for separating the linear butene isomers. The use of DDR for this purpose is discussed by W. Zhu, et al., “Selective adsorption of unsaturated linear C4 molecules on the all-silica DD3R,” Phys. Chem. Chem. Phys. 2, 1773–1779 (2002). Their experiments indicate that only trans-2-butene is able to diffuse into the structure, while 1-butene and cis-2-butene are excluded. The key component, 1-butene, is not recovered as a pure component and the duration of the associated adsorption/desorption cycles for recovering trans-2-butene are likely long due to its low rate of diffusion into the material.
The advantages of reactivity control and size exclusion afforded by materials like DDR may not optimally meet all the necessary requirements for an efficient separation process. The window size also has to be optimally controlled such that short duration cycles are achieved.